A physical quantity sensor having a beam structure formed in a semiconductor substrate is disclosed in, for example, Japanese Patent Application Publication No. H06-349806. The beam structure is formed in the substrate by a micro machining method. As shown in FIG. 19A, a SOI substrate having a silicon substrate 200, an embedded oxide film 201 and a silicon layer 202 is prepared. The silicon layer 202 is formed on the silicon substrate 200 through the embedded oxide film 201. A groove 210 is formed in the silicon layer 202. The groove 210 extends in a vertical direction of the substrate 200, and reaches the embedded oxide film 201. Then, a part of the embedded oxide film 201 disposed under a movable portion 211 is removed completely by a sacrifice layer etching method. Thus, the movable portion 211 becomes movable. Further, another part of the embedded oxide film 201 disposed under the fixed portion 212 remains so that the fixed portion 212 is supported on the substrate 200 through the embedded oxide film 201.
However, as shown in FIG. 19B, the other part of the embedded oxide film 201 disposed under the fixed portion 212 is partially removed when the part of the embedded oxide film 201 disposed under the movable portion 211 is removed completely. Therefore, it is required that the width W1 of the fixed portion 212 is sufficiently wider than the width W2 of the movable portion 211 when the fixed portion 212 and the movable portion 211 are designed. The widths of the fixed and the movable portions 211, 212 are determined by using an etching rate of the oxide film 201. Thus, the oxide film 201 remains under the fixed portion 212.
Thus, it is difficult to form the movable portion 211 and the fixed portion 212 with high accuracy. Further, design degree of freedom of the sensor is limited.